This invention relates to electrochemical cells and batteries, and more particularly, to such cells and batteries having lithium-based active material.
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have included an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between the positive and negative electrodes. The electrolyte provides transport of electrolyte positive ions and separates the positive and negative electrodes from one another. Thus, the designation electrolyte/separation is used. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. During use of the cell, lithium ions (Li+) are transferred to the negative electrode on charging. During discharge, lithium ions (Li+) are transferred from the negative electrode (anode) to the positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Cells having metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemically active material of the cathode whereupon electrical energy is released. During charging, the flow of lithium ions is reversed and they are transferred from the positive electrode active material through the ion conducting electrolyte and then back to the lithium negative electrode.
The lithium metal anode has been replaced with a carbon anode, that is, a carbonaceous material, such as non-graphitic amorphous coke, graphitic carbon, or graphites, which are intercalation compounds. This presents a relatively advantageous rechargeable lithium battery in which lithium metal is replaced with a material capable of reversibly intercalating lithium ions, thereby providing the xe2x80x9crocking chairxe2x80x9d battery in which lithium ions xe2x80x9crockxe2x80x9d between the intercalation electrodes during the charging/discharging/recharging cycles. Such lithium metal free cells may thus be viewed as comprising two lithium ion intercalating (absorbing) electrode xe2x80x9cspongesxe2x80x9d separated by the lithium ion conducting electrolyte/separator. The electrolyte/separator usually comprises a lithium salt dissolved in nonaqueous solvent or a mixture of such solvents. Numerous such electrolytes, salts, and solvents are known in the art.
Preferred positive electrode active materials include LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, prepared by reacting generally stoichiometric quantities of a lithium-containing compound and a manganese containing compound. The lithium cobalt oxide (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LiCoO2 is less than the theoretical capacity because less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes (fades) on every successive cycle of operation. The specific capacity for LiMn2O4 is at best 148 milliamp hours per gram. As described by those skilled in the field, the observed reversible capacity is on the order of 60% of the aforesaid value. Obviously, there is a tremendous difference between the theoretical capacity (assuming all lithium is extracted from LiMn2O4) and the actual capacity when much less than one atomic unit of lithium is extracted as observed during operation of a cell.
Capacity fading is calculated according to the equation given below. The equation is used to calculate the first cycle capacity loss. This same equation is also used to calculate subsequent progressive capacity loss during subsequent cycling relative back to the first cycle capacity charge reference.             ((      FC       charge capacity)        -                  (        FC         discharge capacity))            xc3x97      100      ⁢              xe2x80x83            ⁢      %            FC     charge capacity  
In view of the present state of the art, there remains the difficulty of utilizing lithium manganese oxide based electrode materials over an extended cycle life due to the disadvantage of significant capacity loss on progressive cycling.
The present invention provides a method for cycling a battery containing lithium metal oxide active material, such as lithium manganese oxide (LMO) in a cell in a manner which reduces the extent to which loss of capacity occurs. The method of the invention is conducted by performing at least one initial conditioning charge and discharge cycle wherein the initial conditioning charge is preferably to essentially the design voltage of the battery and discharge is preferably to essentially the full depth of discharge; charging the conditioned battery preferably to essentially the full charge, and then discharging the battery to greater than 90% and less than 100% of the full depth of discharge. In a preferred embodiment of the invention method, the charge and discharge cycles are repeated in sequence, preferably up to four conditioning cycles. The cycling regime after conditioning is to depth of discharge greater than 95% and less than 99% of the full depth of discharge.
According to another feature of the invention, the step of discharging the charged battery is conducted to a discharge voltage greater than about 3.4 volts at a temperature of about 50xc2x0 C. or more. Alternatively, the discharge step is conducted to a discharge voltage greater than about 3.6 volts and at a temperature of about 50xc2x0 C. or more.
According to a further aspect of the invention, the conditioned battery is charged at a charge rate of C/2, and a discharge rate of C/5 is used. This charge/discharge cycling regime provides a greatly increased cycle life over the traditionally used C/2 charge and C/2 discharge rates.